Flickr and Twitter filters using oEmbed and JSON

[jbhannah]

Added a filter using Flickr's oEmbed service and JSON response to embed images from supported pasted Flickr URLs (http://www.flickr.com/photos/* or http://flickr.com/photos/*). Maximum width and height arguments and link options (target, rel, &c.) can be passed to the filter:

auto_html_for :body do
  flickr(:maxwidth => 300, :maxheight => 300, :link_options => { :target => "_blank" })
end

Likewise, added a filter using Twitter's oEmbed API that uses the pasted URL of a single tweet and accepts any options that can be passed to the API (this fixes issue #6):

auto_html_for :body do
  twitter(:align => :center)
end

[jbhannah]

Updated with Twitter filter.